Liquid crystal display devices are display devices utilizing a liquid crystal composition for display. A typical display mode thereof is applying voltage to a liquid crystal composition sealed between paired substrates to change the alignment state of liquid crystal molecules in the liquid crystal composition according to the applied voltage, thereby controlling the amount of light transmission. These liquid crystal display devices, having characteristics such as thin profile, light weight, and low power consumption, have been used in a broad range of fields.
When voltage is not applied, the alignment of liquid crystal molecules is generally controlled by an alignment film disposed on the surface of a substrate. Conventionally used materials (liquid crystal aligning agents) for alignment films are polyimide and polyamic acid, which is a precursor thereof, in many cases. Polyimide (polyamic acid) exhibits better physical properties such as heat resistance, affinity with liquid crystal, and mechanical strength than other organic resins. However, more reliable alignment films are demanded in association with, for example, improvement of the performance of liquid crystal panels, expansion of the range of use thereof, and diversification of the use environment thereof. Liquid crystal display devices including a polyimide-based (polyamic acid-based) alignment film may suffer generation of display defects such as image sticking and stain in some cases, and they are likely to occur especially due to storage in a high-temperature environment.
Considering this situation, in order to suppress reduction in display quality of liquid crystal display devices in a high-temperature environment, Patent Literature 1 discloses a polybenzoxazole-, polybenzothiazole-, or polybenzimidazole-based alignment film which has higher thermal stability than conventional polyimide-based (polyamic acid-based) alignment films. The literature also discloses that a thermally stable photo-alignment film is obtainable by introducing a photo-active group into the material of such a polybenzoxazole-, polybenzothiazole-, or polybenzimidazole-based alignment film. However, even the alignment film disclosed in Patent Literature 1 fails to have sufficient thermal stability. In the IPS mode and the FFS mode, this alignment film may cause impairment of the contrast and the long-term reliability. Such impairment is significant especially when a horizontal photo-alignment film and a negative liquid crystal material are combined.